Mouse embryos continuously differentiate in vitro from the two-cell egg stage to the early somite stage, which is equivalent to an embryo of 9 days' gestation. Up to the stage of the early somite, the development of the mouse embryo relies on the yolk sac placentation. Failure in developing chorioallantoic circulation in vitro beyond the stage of the early somite causes autolysis of the mouse embryo. The goal of this proposal is to (1) study the in vitro morphogenesis of the chorioallantoic placenta from the blastocyst, and (2) to study the teratocarcinogenesis of mouse embryos in vitro with or without chemical carcinogens.